The present invention relates to DWDM channel filters, and more particularly to DWDM channel filters based on a Bragg grating that allows the filter function to be parked between DWDM channels.
It is known that wavelength division multiplexing (WDM) uses different wavelengths (or wavelength bands) of light to carry information along an optical fiber, each wavelength band representing a different channel. Each channel has a predetermined bandwidth and the channels are each spaced a predetermined distance apart (based on the stability, dispersion and other optical characteristics of the network). As the demand for bandwidth increases, the spacing between channels becomes closer to create a dense wavelength division multiplexed (DWDM) system. For example, for a 200 GHz system, the spacing between the center of two adjacent wavelength channels is about 1600 picometers (or 1.6 nanometers), for a 100 GHz system, the spacing is about 800 picometers, for a 50 GHz system, the channel spacing is about 400 picometers, and for a 25 GHz system, the channel spacing is about 200 picometers. Also, the width of the channel is dependent on the data rate of the signals carried on each channel.
As is known, fiber Bragg gratings can be used as effective filter elements in a range of applications in WDM and DWDM optical communications networks. For example tunable grating filters may be used in optical add drop multiplexers, such as those described in U.S. Pat. No. 6,020,986, to Ball, entitled xe2x80x9cProgrammable Add-Drop Module for use in an Optical Circuitxe2x80x9d, U.S. Pat. No. 5,726,785, to Chawki et al, entitled xe2x80x9cOptical Add-Drop Multiplexer Using Optical Circulators and Photoinduced Bragg gratingsxe2x80x9d, U.S. Pat. No. 5,748,349, to Mizrahi, entitled xe2x80x9cGratings Based Optical Add-Drop Multiplexers for WDM Optical Communications Systemxe2x80x9d, and U.S. Pat. No. 5,579,143, to Huber, entitled xe2x80x9cOptical System with Tunable In-Fiber Gratingsxe2x80x9d.
Such patents teach reflecting or blocking a channel using a fiber Bragg grating filter by tuning the Bragg grating reflect the channel and/or allowing a channel to pass by tuning the Bragg grating to be between channels, e.g., xe2x80x9cparkingxe2x80x9d the grating between channels to be out of the way and not reflect the channel being passed.
An object of the present invention is to provide a tunable Bragg grating that provides for filtering WDM and DWDM channels and provides for parking the gratings filter function between such channels.
In accordance with an embodiment of the present invention, a tunable filter parking device comprises an optical waveguide that includes an outer cladding disposed about an inner core. A portion of the cladding includes a tapered region wherein the cladding has a minimum cross-sectional dimension of 0.3 mm. A chirped grating is written in the inner core of the tapered region of the cladding.
In accordance with another embodiment of the present invention, a tunable optical filter for dropping an optical channel from an input signal having a plurality of input channels is provided. The optical filter comprises a plurality of concatenated tunable filter parking devices. Each tunable filter parking device includes an optical waveguide having an outer cladding disposed about an inner core. A portion of the cladding includes a tapered region wherein the cladding has a minimum cross-sectional dimension of 0.3 mm. A chirped grating is written in the inner core of the tapered region of the cladding. A light directing device directs the input signal to the tunable filter parking device and directs at least one drop signal that is reflected from at least one of the tunable filter parking device to an output port.
In accordance with another embodiment of the present invention, an optical add/drop multiplexer for dropping and/or adding an optical channel of an input signal having a plurality of input channels is provided. The optical add/drop multiplexer comprises a plurality of concatenated tunable filter parking devices. Each tunable filter parking device includes an optical waveguide having an outer cladding disposed about an inner core. A portion of the cladding includes a tapered region, wherein the cladding has a minimum cross-sectional dimension of 0.3 mm. A chirped grating is written in the inner core of the tapered region of the cladding. A first light directing device directs the input signal to the tunable filter parking devices and directs at least one reflected drop signal to a drop port. A second light directing device directs an add signal to the tunable filter parking devices and directs at least one reflected add signal to an express port.